Abstract:
Data set of approximately 10,000 shallow focus (depth <200 km), teleseismic (epicentral distance 30° to 90°) earthquakes, for the period 1990-2007, from 16 Indian and 3 foreign seismic stations have been used in a study of P wave arrival station residuals, including azimuthal effects. The events were selected from EHB Bulletin and were used to compute P arrival station corrections.
Station corrections are provided for 19 seismic stations. The station corrections involve three terms: the static effect and two cosine terms with appropriate phase shifts.
An attempt is made to understand the observed result of the direction of the first and second azimuthal terms. The idea was to relate the slow moving direction for the second azimuthal term to mantle anisotropy and stress direction in the crust similar to the analysis carried by others.
The azimuth independent term suggests if the upper mantle and crust in the vicinity of the station is slow or fast, thus resulting in late or early P arrivals at the station, respectively.
The first and second azimuthal terms exhibit regional consistency over geographic areas. The cos20 terms give the direction of the upper mantle anisotropy as expected.
